The present invention relates generally to methods and apparatus for use in well completion operations, and more particularly, relates to annulus pressure fire mechanisms for perforating guns. More particularly, the invention relates to a tubing actuated firer and to a method of releasing the tubing to drop the gun.
Oil and gas wells are generally completed by lowering a perforating gun into a cased borehole to perforate the well by shooting perforations through the casing, cement and into the hydrocarbon formation to permit the hydrocarbons to flow into the cased borehole and up to the surface. U.S. Pat. Nos. 3,706,344 and 4,484,632 to Vann, disclose suspending a packer and perforating gun on a tubing string, setting the packer to isolate the production zone, releasing the trapped pressure below the packer by opening the tubing string to fluid flow, actuating the perforating gun through the tubing string, and immediately producing the well through the tubing string upon perforation. One means for actuating the perforating gun includes dropping a bar through the tubing string to impact the firing head of the perforating gun.
However, after a borehole has been drilled into the ground and the casing cemented into position, well fluids fill the cased borehole with drilling mud and debris which gravitate toward the lower end of the cased borehole and tend to densify and congeal into a heavy layer of material. Additionally, debris such as rust, sand, scale, and other material dropped into the well from the surface also tend to collect and densify toward the bottom of the well. This material often collects around the firing head of the perforating gun.
In a perforating gun having a bar actuated gun firing head, it is possible for such contaminants to densify and collect such that the firing head cannot be sufficiently impacted to detonate the perforating gun. If the bar is unable to sufficiently strike the firing mechanism, the gun will not be detonated. The problem of debris and contamination is compounded when the string is left downhole for a substantial length of time.
In an effort to overcome these problems associated with bar actuated firing mechanisms, systems such as those disclosed in U.S. Pat. No. 4,484,632 to Vann were developed. These systems use an annulus pressure firing mechanism. In an annulus pressure firer, a pressure responsive means is disposed in the pipe string above the packer and is connected by means of a cable or a tube containing hydraulic fluid to the firing head on top of the perforating gun. With this system, the gun is actuated by a differential pressure created between the interior of the tubing and the well annulus. This eliminates the need to run a bar or other mechanism down the tubing to actuate the gun, which mechanism could be hindered by the collection of mud or other debris.
While the annulus pressure fire mechanism was an advancement over many of the previous designs, it still possesses certain problems. For example, the cable or tubing connecting the pressure responsive means above the packer with the firing head often prevented the lowering of other tools and equipment such as logging tools down the tubing to the top of the firing head. Rather, these tools could generally only be lowered as far as the pressure responsive means which was located above the packer. Accordingly, what is needed in the art is an annulus pressure firer mechanism which allows the tubing to remain opened such that other tools can be lowered down to the firing head.
After a perforating gun has been lowered into a well, it is often desirable to separate the gun from the end of the tubing and drop it to the bottom of the well. This is often necessary to drop the gun if a logging tool or sampling equipment needs to be lowerd through the tubing to a point below where the gun is located. Also, during production it is often desirable to drop the gun to completely open the end of the tubing to allow the oil and gas to flow unimpeded into the tubing.
Accordingly, it would be desirable to have a release mechanism in conjunction with an annulus pressure fire mechanism which would allow the tubing to be separated above the perforating gun such that the gun could be dropped to the bottom of the well.
The present invention provides apparatus and methods which can accomplish these functions.